5.3 Given a scenario, troubleshoot common issues with network services
Switching Issues
📘CompTIA Network+ (N10-009)
1. What is a VLAN?
A VLAN (Virtual Local Area Network) is a way to logically separate a physical network into multiple smaller networks. Even if devices are plugged into the same switch, VLANs can make them behave as if they are on completely separate networks.
- Purpose:
- Improves security (separates sensitive traffic).
- Reduces broadcast traffic (makes network more efficient).
- Helps organize networks logically, not just physically.
Example:
You have a company network with HR, IT, and Sales. Even if all employees are connected to the same switch, HR can be on VLAN 10, IT on VLAN 20, and Sales on VLAN 30. Devices in HR cannot directly talk to IT or Sales unless routed through a router or Layer 3 switch.
2. Common VLAN Misconfigurations
VLAN misconfigurations are mistakes in setting up VLANs that prevent proper communication or cause network problems. Let’s go through the main ones:
a. Incorrect VLAN assignment
- Problem: A port is assigned to the wrong VLAN.
- Effect: Devices connected to that port cannot communicate with the devices in the correct VLAN.
- IT Example: An IT technician’s computer is accidentally placed in VLAN 30 (Sales) instead of VLAN 20 (IT). They can’t access IT servers.
Exam tip: Check port VLAN assignments using commands like:
show vlan brief
b. VLAN not configured on the switch
- Problem: The VLAN exists on one switch but not on the other switches that need it.
- Effect: Devices on different switches in the same VLAN cannot communicate.
- IT Example: HR computers on Switch A are in VLAN 10, but Switch B doesn’t have VLAN 10 configured. HR devices on Switch B can’t communicate with those on Switch A.
Solution: Make sure the VLAN exists on all switches and is active.
c. Trunk misconfigurations
- Problem: Trunks are used to carry multiple VLANs between switches. If a trunk is misconfigured, some VLANs might not pass through.
- Effect: Devices on different switches in the same VLAN cannot communicate.
- IT Example: VLAN 20 (IT) is not allowed on the trunk link between Switch A and Switch B. IT devices on different switches cannot share files.
Exam tip: Check trunk configuration with commands like:
show interfaces trunk
d. Native VLAN mismatch
- Problem: Each switch trunk has a native VLAN (default VLAN for untagged traffic). If switches have different native VLANs, miscommunication occurs.
- Effect: Traffic may be sent to the wrong VLAN, causing network confusion and security risks.
- IT Example: Switch A has native VLAN 1, but Switch B has native VLAN 99. Untagged traffic from one switch goes to the wrong VLAN on the other switch.
Solution: Standardize the native VLAN across all trunk links.
e. Overlapping VLANs
- Problem: Two different VLANs use the same VLAN ID across different parts of the network but are intended for separate groups.
- Effect: Devices may communicate unintentionally or lose connectivity.
- IT Example: VLAN 20 is used for IT in one department and for Sales in another. Traffic may mix, causing confusion and security issues.
3. How to Troubleshoot VLAN Misconfigurations
- Check port VLAN assignment
- Use commands like:
show vlan brief- Verify the correct VLAN for each port.
- Check trunk links
- Ensure trunks are configured to allow the correct VLANs.
- Verify with:
show interfaces trunk - Verify VLAN existence on all switches
- Make sure all VLANs are created and active.
show vlan - Check native VLAN settings
- Ensure native VLAN matches on all trunk links.
show interface trunk - Test connectivity
- Ping devices in the same VLAN across switches. If it fails, trace VLAN configuration.
4. Exam Tips
- VLAN issues are very common in troubleshooting scenarios. Focus on:
- Port assignments (access port vs trunk port).
- VLAN IDs matching across switches.
- Trunk configuration (which VLANs are allowed).
- Native VLAN consistency.
- Device connectivity tests.
- Remember, if devices can’t talk, it’s often a VLAN problem.
5. Summary in Simple Words
Think of VLANs as separate “chat rooms” on a switch.
- If a device is in the wrong chat room (wrong VLAN), it can’t talk to the intended people.
- If the hallway between chat rooms (trunk link) blocks certain rooms, people can’t meet even if they are in the right room.
- If rooms are misnamed or missing, confusion happens, and communication fails.
The key to passing the exam: know how to spot and fix these VLAN misconfigurations quickly.
